This research will examine the behavioral effects of smoked marijuana in moderate marijuana smokers. A variety of behavioral and physiologic variables will be measured, including heart rate, subjective mood states, psychomotor performance, and cognitive function. A novel aspect of the proposed research will be use of expired air carbon monoxide level as a quantitative index of marijuana smoke inhalation. Plasma levels of Delta9 THC will also be determined and correlated with CO levels and other drug effects. Two proposed studies relate to marijuana self-administration. The first will examine the effect of cigarette THC concentration on marijuana smoking behavior by determining whether marijuana smokers adjust their intake of THC when smoking cigarettes containing different levels of THC. The second study will examine the discriminative stimulus properties of smoked marijuana by training smokers to discriminate placebo from active marijuana. This procedure could prove useful for screening new compounds for marijuana-like subjective effects, and possibly dependence potential. A third study will evaluate a cumulative dosing procedure for obtaining rapid, replicable dose-effect functions for smoked marijuana. Such a procedure would allow investigators to obtain dose-effect functions in a single session, which would be very useful for future studies examining changes in sensitivity to smoked marijuana (e.g., studies of tolerance and cross-tolerance, sensitization, and drug interactions). The final study will determine whether marijuana smoking produces residual "hangover" effects the day after smoking. Although there is some anecdotal and experimental evidence for the existence of hangover effects after marijuana smoking, there have been no studies explicitly designed to address this issue. This study will help to characterize the nature of any observed hangover effects and the dosing conditions under which they develop. Such hangover effects may have important and as yet unrecognized public health implications.